Lord Indra's Brahmahatya Sin

Srimad Bhagavatam - Indra's Brahmahatya Sin 

Once in the Court of Indra, gods and semi-gods were entertaining Indra. For a moment, rajoguna (representing ego) overtook Indra's senses.

Just then revered Guru Brihaspati came to the Court, but Indra ignored him. Brihaspati felt humiliated and left the court silently.

Quickly realizing his mistake, Indra went in search of his preceptor to seek pardon. Bruhaspati, knowing Indra's mind, disappeared using his yogic power and could not be found anywhere.

With no preceptor, devas were in deep trouble and lost their strength. Seizing this opportunity, demons with the help of Shukracharya, attacked the devas and were successful in driving out devas from devaloka.

Devas sought Brahma's help to overcome the situation. Brahma expressed his disappointment over Devas’ behaviour towards their preceptor. He said disrespecting one's guru is disastrous. None can save from this sin.

Brahma asked Devas to approach highly revered Sage Vishwarupa, son of Twasta, to be their preceptor. Having no other option, the devas approached Viswarupa. Though younger in age, devas accepted him because he possessed superior knowledge. 

Viswarupa taught devas ‘Narayana astra’ (a protective divine mantra reciting it in a prescribed manner acts as an armour against enemies). Using it, Devas were able to win over the demons and regain the lordship of 3 worlds.

Viswarupa had 3 heads. He was partaking Soma (a juice obtained from yajna) using first mouth, Sura (local wine) with second, and regular food using the third mouth. Viswarupa's father belongs to Devas's lineage, while his mother is from demon lineage. Viswarupa, hence, was not indifferent to demons. 

Being an official priest of Devas, he duly offered a soma drink to Devas. Without anyone's knowledge, he offered the same to demons as well. Demons, thereby, were becoming stronger. This treacherous act of Viswarupa was made known to Indra.

Indra, for the good of all, immediately killed Vishwarupa. By killing his preceptor, Indra earned brahmahatya sin. 

The head by which Vishwarupa drank Soma became Kapinjala Bird (Barli Siplae hakki/ Francolin), that by which he drank wine became Kalvinka (sparrow) and that by which he ate food was converted into Patridge.

Indra accepted wholeheartedly the sin of killing a Brahmana. At the end of the year, he shared it with the earth, water, trees, and women.

Source: Bhagavatha Mahapurana